1. Field
The presently disclosed embodiments relate generally to laser control, and more specifically to laser performance compensation for aging and temperature changes.
2. Background
Lasers are increasingly used in systems such as high speed communications links, fiber optic channels and medical diagnostics. Market trends demand increased levels of reliability and performance in laser systems. Lasers having signals with accurate output power and signal amplitude are required to meet these performance demands.
Individual lasers exhibit significant variations in performance characteristics when they are newly manufactured. Additionally, all lasers substantially degrade in performance with age and changes in temperature. Performance degradation causes a reduction in output power and signal strength, resulting in decreased Signal to Noise Ratios (S/Ns) and Extinction Rates, as well as increased Bit Error Rates (BERs).
Communications receivers require that signals maintain acceptable signal strength and reliable operating parameters. In order to generate transmission signals that meet receiver requirements, lasers must be adjusted to compensate for individual variations and performance degradations occurring over time.
Various conventional methods are used to compensate for changes in laser performance characteristics. Conventional performance compensation methods have drawbacks such as communication disruption and non-optimal output power adjustments. Non-optimal power adjustments may produce inaccurate output signals that are difficult to receive, and frequently overdrive the laser, reducing its life.
Before adjustments can be made for performance degradations caused by aging and temperature changes, output power and temperature must be accurately monitored. The output power of many lasers available today is monitored with photodiodes that are integrated with the laser in a single package. The photodiodes may also be a component of an integrated circuit that is associated with the laser's driver or a Vertical Cavity Surface Emitting Laser (VCSEL) array. For economic reasons, it is common to utilize very slow photodiodes for monitoring the laser output. In some cases the photodiodes exhibit a frequency response that is several orders of magnitude lower than the frequency response of the laser. Photodiodes with frequency responses that are slower than the lasers they monitor can reliably measure the laser's average power output, but pose a problem in determining the amplitude of the optical pulses for transmitting information. The amplitudes of optical pulses cannot be measured because the photodiode will not generate significant output in response to the Alternating Current (AC signal) output representing data transmission.
In digital communications, it is necessary to monitor the amplitude of the optical pulses in order to distinguish the transmission of a logical one from the transmission of a logical zero. In both analog and digital communications, the magnitude of the optical signal represents the strength of the signal and has a direct impact on signal to noise ratio and transmission reliability. Because sensing power output with low frequency response photodiodes permits only the average power of the laser, rather than the amplitude of data transmission light pulses to be monitored, accurate power output feedback information is not available to adjust the magnitude of optical pulses representing the data. Without accurate amplitude feedback information, output power cannot be properly controlled, causing the Optical Modulation Amplitude, Extinction Ratio and BER to degrade with temperature changes as well as aging.
To perform an accurate power measurement with a slow photodiode, an input power signal must be maintained at a fixed power level causing the system to produce a constant value of light output, which is always equal to the measurable average power value. Since the slow photodiode can't be relied upon to determine the output power of a high frequency signal, other methods have been employed. For example, one method commonly used consists of applying a signal with known amplitude to the laser transmitter while a measurement is made of the resulting output power with an instrument instead of a photodiode. The measurement instrument used is one that can respond to high frequency of light power transitions. This procedure disrupts the signal transmission preventing the transmitter from sending information over the communications channel while adjustments are carried out. Disruption in communication is contrary to the goals of high reliability and 100% up-time in present systems.
Another example of an intrusive power adjustment method is an approach that relies on the application of a tone signal to the laser. The tone is recovered by the monitor photodiode and the recovered signal used to determine changes in laser performance. This method is disadvantageous because, again, the tone disrupts the transmitted signal because the magnitude of the tone signal is of similar magnitude to the magnitude of the transmitted signal. Disruption also causes a significant reduction of the noise margin, which renders this approach inaccurate.
Temperature sensors are commonly utilized to determine when performance adjustments are appropriate due to changes in temperature. Conventional reliance on temperature sensors is also problematic. Temperature sensors, unlike photodiodes, are not commonly integrated with laser or driver devices. The temperature sensor must be mounted at a location external to the laser itself, producing a measurement that is poorly correlated to the actual operating temperature of the laser. The problem is then compounded when inaccurate temperature measurements are used as indexes to determine power adjustments from equally unreliable look-up tables.
Look-up tables are created at the factory for each laser manufactured. Each laser must have its own look-up table because the performance characteristics of each unit differ with variations in constituent parts and manufacture. This method of creating temperature lookup tables requires a costly process on the production line to heat each laser in an environmental chamber at incremental temperatures. Large numbers of test temperature samples produced by small temperature increments, which are necessary for accurate interpolation, increase the production cost. The table is populated with a bias and modulation current for each temperature tested, unique to the particular laser. Even this labor-intensive effort cannot produce an accurate table because the table cannot compensate for aging. Aging cannot be predicted ahead of time with the required level of individual precision to create a table of aging values for a given laser. In some cases, Manufacturers resort to tightening the performance specifications for the laser system so it will still perform adequately after aging degradation. The result of the tightening of the specification is a lower manufacturing yield for the components used in the laser system, which increases costs.
Conventional methods of compensating for degradations in laser output power are inadequate because temperature and output power measurement methods rely on external physical devices that produce inaccurate feedback information. Costly labor intensive look-up tables do not produce reliable results because temperature indexes are poorly correlated to actual laser operating temperatures, and the effects of aging cannot be accurately predicted for individual lasers. Thus, there is a need in the art for improved methods of laser performance monitoring and compensation, which do not employ external measurement components and inaccurate lookup tables or disrupt transmitted data throughput.